1. Field of the Invention
The invention in general relates to electrosurgery, and in particular to an electrosurgical unit that produces a high power output at the electrosurgical electrodes.
2. Description of the Prior Art
Electrosurgical generators which produce high frequency electric currents for cutting of tissue and coagulation of small blood vessels have been well-known for several generations. It has also been well-known that different types of waveforms produce different types of electrosurgical functions. In modern electrosurgery, these functions are generally broken down into two primary functions, coagulation and cut, with coagulation being a process in which small blood vessels are sealed by controlled electrical cauterization, and cut being a process in which tissue and vessels are severed by electrical energy. Generally these two functions or modes, cut and coagulation, are broken down into two further modes commonly called fulgration and blend. Fulgration is a form of coagulation in which the electrode is spaced from the tissue to be coagulated and sprays electrical energy over a large area of tissue, in contrast to "pure" coagulation in which the electrode is touched to the tissue to coagulate the tissue which the electrode touches. Blend is a mode which combines both cut and coagulation, therefore allowing the surgeon to cut and coagulate at the same time. It is known that the desirable output power and voltage at the electrodes for a cutting or coagulation procedure varies depending upon the depth of cut, the impedance provided by the tissue and other substances between the electrodes, and numerous other factors. Thus, electrosurgical units have been designed to provide various means of controlling power and the amplitude of the output voltage at the electrodes. Generally, two approaches have been taken to provide the appropriate power and voltage control. In one approach, separate electronic circuits inherently capable of producing different power and voltage levels are included in one electrosurgical unit and switches are used to connect the different circuits to the electrodes. See, for example, U.S. Pat. Nos. 3,952,748 issued to Paul L. Kaliher, et al. and 3,964,487 issued to Donald W. Judson. In another approach, a voltage representative of the output voltage is sensed and is fed back into the RF signal generation circuitry to obtain a "constant" output voltage under changing load conditions. See, for example, U.S. Pat. Nos. 4,092,986 issued to Max Schneiderman, 3,875,945 issued to Joshua Friedman and 4,126,137 issued to G. Kent Archibald. The former approach is quite effective, but results in a complex and bulky electrosurgical generator. It also requires switching between the coagulation and fulgration modes, which is often inconvenient, since surgeons may desire to switch back and forth between the two modes rapidly, and often. The latter approach does not yield a satisfactory output voltage for all functions and conditions desired by a surgeon, and is relatively inefficient, resulting in more heat dissipation from the unit than is desirable.
It is known in the art to employ transformers in coupling power to the electrodes. Electrosurgical units have employed conventional transformers in which the windings of the primary and secondary are overlaid.